The present invention relates generally to solar or photovoltaic cells and, more particularly, relates to thin film solar cells deposited onto plastic substrates at low temperatures.
The conversion of the sun""s electromagnetic energy into electricity occurs in a wide variety of semiconductor materials. Single element semiconductors, such as silicon (Si) and germanium (Ge), as well as some compound materials, such as gallium arsenide (GaAs), cadmium telluride (CdTe) and indium phosphide (InP), are capable of forming junctions that create a built-in electric field in the material. These materials are further capable of absorbing photons of energy sufficient to create electron-hole pairs, thereby creating an electrical current. This phenomenon, which is known as the photovoltaic (PV) effect, is the scientific and theoretical foundation for today""s solar energy market.
Electricity produced by solar cells would have a bigger impact were it not so expensive. The cost of the raw materials, equipment, and energy required to manufacture crystalline and polycrystalline solar cells is prohibitive for wide-scale market penetration. Presently, it is cheaper to produce electricity with coal fired or nuclear plants. As much as half the cost of solar cells lies in an expensive substrate such as crystalline silicon. A less expensive substrate is desirable.
The arrangement of cells into solar modules or arrays creates enormous industry-wide manufacturing problems. The cost and complexity of conventional solar panels conspire against the deployment of solar cells in many applications, such as portable electronics and computers. Crystalline thick-film solar cells are also fragile and bulky, which minimizes their acceptability in the marketplace. They are less rugged than conventional materials such as roof tiles. They are expensive to transport because they must be carefully protected from breakage. A flexible solar cell, if available, would be more rugged, less likely to break, and could be rolled up for easy transport or shipping.
Thin-film solar cells have been explored as a solution to this dilemma. Thin-film solar cells are several orders of magnitude thinner than their single-crystal and thick-film polycrystalline counterparts and thin-film processes lend themselves to amorphous construction. An amorphous thin-film cell can be manufactured in a fraction of the time that it takes to monitor and incubate crystal growth. Energy-intensive processes such as chemical treatments and high-temperature annealing are not necessarily required for the manufacture of amorphous materials. All polycrystalline CdTe solar cell manufacturing processes urge heat treatment, either before or after deposition, at temperatures of at least 400 degrees centigrade. Amorphous CdTe is incompatible with such heat treatments.
The present invention addresses the drawbacks of prior art solar cells noted above and provides a flexible, thin film solar cell.
In one embodiment of the invention, a solar cell comprises a flexible substrate; an n-type window layer deposited onto the substrate at a temperature sufficiently low so as not to damage the substrate; and a p-type absorption layer deposited onto the n-type window layer at a temperature sufficiently low so as not to damage the substrate. In this embodiment, the deposition is preferably carried out at a temperature sufficiently low such that crystallization of the n-type and p-type layers does not occur.
In another embodiment of the invention, a thin film flexible solar cell comprises a plastic or polymer substrate; a thin film of n-type cadmium sulfide deposited onto the substrate at a temperature sufficiently low so as not to damage the substrate; and a comparatively thicker film of p-type cadmium telluride deposited onto the n-type window layer at a temperature sufficiently low so as not to damage the substrate. In this embodiment, the cadmium sulfide and cadmium telluride films have either an amorphous or a polycrystalline structure.
In a further embodiment of the invention, a thin film flexible solar sell comprises a flexible substrate; a current collection layer deposited onto the substrate; an n-type semiconductor film that is deposited onto the current collection layer and has an amorphous atomic structure; a p-type semiconductor film having a bandgap energy less than the n-type semiconductor film that is deposited onto the n-type semiconductor film and has an amorphous atomic structure; and a metallization layer deposited onto the p-type semiconductor film.
In a still further embodiment of the invention, a method for manufacturing a thin film flexible solar cell is provided. It comprises the steps of providing a plastic or polymer substrate; depositing a layer of an n-type semiconductor on the substrate at a temperature sufficiently low to avoid melting or damaging the substrate; and depositing a layer of a p-type semiconductor on the substrate at a temperature sufficiently low to avoid melting or damaging the substrate. In this embodiment, the deposition is preferably carried out at a temperature such that the semiconductor layers have an amorphous or polycrystalline structure